Blue Laser Projection Printing of Conductive Complex 2D and 3D

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Functional Inorganic Materials and Devices

Blue Laser Projection Printing of Conductive Complex 2D and 3D Metallic Structures from Photosensitive Precursors Xiaolu Wang, Kejian Cui, Qin Xuan, Caizhen Zhu, Ning Zhao, and Jian Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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ACS Applied Materials & Interfaces

Blue Laser Projection Printing of Conductive Complex 2D and 3D Metallic Structures from Photosensitive Precursors Xiaolu Wang,†,‡ Kejian Cui,‡ Qin Xuan,‡,§ Caizhen Zhu,† Ning Zhao,*,‡ and Jian Xu*,‡,§

†Institute

of Low-dimensional Materials Genome Initiative, College of Chemistry and

Environmental Engineering, Shenzhen University, Shenzhen, Guangdong, 518060, P. R. China. ‡Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China. §University of Chinese Academy of Sciences, Beijing 100049, P. R. China

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ABSTRACT: Photosensitive precursors are developed for the printing of 2D and 3D conductive structures via blue laser projection printing. With the assistance of photosensitizer, metal nanoparticles can be efficiently photosynthesized under laser irradiation of low light intensity (45-290 mW cm-2). By projecting well-defined laser patterns on the precursor, corresponding 2D metal structures with the finest line of about 50 μm can be formed on various substrates including flexible polymer thin films, curved substrate and ground glass. Moreover, complex 3D objects with nanoparticles embedded in the polymeric matrix are constructed via 3D printing combining photoreduction of metal precursor and photopolymerization of resin. The as-prepared structures exhibit promising conductivities after sintering (in the order of magnitude of 106 S m-1). A possible mechanism of photochemical synthesis of metal nanoparticles upon exposure to blue laser is proposed. The high efficiency and low cost of the technique, the complexity of the structures prepared, and the applicability to various substrates and metals (including silver, gold and palladium) promise practical applications of this approach in printed electronics industry.


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Keywords: photosensitive material, digital light processing, printed electronics, 3D printing, photoreduction


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1. INTRODUCTION Printed electronics are widely used in transistors,1 supercapacitors,2,3 displays,4,5 touch screens6 and so on. In particular, the growing demand of novel electronic devices like that are flexible, large-area, lightweight, wearable and cheap requires improvements in both printing technologies and materials. Metal materials such as silver, gold and copper have good electrical conductivity and printability. They were often used to prepare twodimensional (2D) flexible electronic devices7–12 as well as conductive three-dimensional (3D) objects13–17. Among the many types of metal printing technologies have been developed in recent years,11,18 direct laser writing (DLW)13,15,19 has less limitation on ink viscosity. DLW approaches including selective laser sintering of metal nanopastes13 or metal nanoparticle inks10, and laser direct synthesis and patterning of metal nanoparticles via

in-situ photoreduction of metal precursors9,20 have been exploited. DLW is also a versatile technique to print electronic devices especially those are flexible. The printing processes can be performed under moderate conditions, which are quite suitable for printing metal patterns on flexible polymer substrates that are usually heat-sensitive.8,9,20 However, the


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common laser sintering and photoreduction of metal materials usually require high photon flux which is only available in the laser focal spot.20,21 Meanwhile, the patterning is carried out via a “point-to-point” scanning of the generally micron-sized focal spot within predesigned regions to “write” a pattern. The printing setup of this technique is complicated and it would be quite time-consuming for printing complex and large-area structures.15,16 These problems greatly limit the applications of DLW in flexible electronics. The technical challenge to overcome these limitations is how to improve or change the “point-to-point” scanning approach for better defining laser patterns on metal materials. Currently, digital laser patterns with practical size and high resolution can be directly projected through laser projection, for example by using commercial laser projectors. As being widely used in 3D printing of photocurable resins, a projector is employed to project a mask image on the surface of photocurable liquid resin to selectively solidify a thin layer of the resin with exact shape of the image.22 Following this approach, 3D objects can be created in a “layer-by-layer” manner. Compared to the spot scanning method, it has intrinsic advantage of printing complex structures faster.15,17–21 However, this layered manufacturing principle of utilizing light projection technique was rarely applied to printed


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electronics.26 It is due to the fact that printing the maturely developed materials, such as metal nanoparticle based inks and metal precursor inks, needs high input light intensities.8–10,12,15,21,27 The laser intensities used for DLW are usually higher than 106 mW cm-2. Considering a laser projection printing in an area of 1 cm2, that the required incident power is at least 103 W. Laser projector with such high power is rarely available. Otherwise, the high energy input would lead to a dramatic temperature increase of materials and produce thermal damage to substrates.8,12,28 Therefore, to develop the application of this efficient laser projection printing technique in flexible electronics, novel formulations of metal materials suitable for metal patterning upon laser exposure of low energy have to be exploited. In this work, we successfully developed a highly photosensitive precursor containing metallic salt, photosensitizer and surfactant for printing conductive metal structures by laser projection technique. A portable commercial GaN-based blue laser projector with central wavelength at 445 nm was employed. The applied light intensity is in the range of 45-290 mW cm-2, at least two order of magnitude lower than that usually used for metal printing (Table S1 in Supporting Information).8–10,12,15,21,27 Efficient metal patterning is


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impossible under this irradiation.21,24 In the present work, however, conductive complex 2D metal structures on various substrates including heat-sensitive polymer films were successfully prepared from the precursor via laser projection printing. Moreover, 3D conductive objects of photochemically synthesized metal nanoparticles in polymer matrix were also fabricated by using a blue laser projector equipped 3D printer. The exploited photosensitive metal precursors have intense visible light absorption, and sufficient amount of metal nanoparticles can be synthesized from them although irradiated by laser of low intensity. With these advantages, the materials successfully enable the applications of laser projection in printed electronics. Meanwhile, the laser projection technique offers unique opportunities to print complex 2D and 3D metallic structures in simple and effective way. The technique provides further advantages such as low cost, applicability to various metals and substrates, and as well as high efficiency that will promisingly facilitate the industrial manufacturing of electronic devices. 2. EXPERIMENTAL SECTION 2.1. Materials. Camphorquinone (CQ, 98%), ethyl 4-dimethylaminobenzoate (EDMAB, 98%), potassium tetrachloropalladate (K2PdCl4, 99%) and hexadecyltrimethylammonium


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chloride (CTAC, 99%) were purchased from J&K Scientific Ltd, China. Ndecanoylsarcosine sodium (NDSS, >98%) was purchased from Tokyo Chemical Industry (TCI) in Shanghai, China. Silver nitrate (AgNO3, AR) was purchased from Guangdong Guanghua Sci-Tech Co., Ltd (JHD), China. Ethylene glycol (AR) was purchased from Beijing Chemical Works, China. Hydrogen tetrachlorocuprate (III) hydrate (99.8%-Au) (49% Au) was purchased from Strem Chemicals, USA. All chemicals were used as received without further purification. Commercially available poly(ethylene terephthalate) (PET), polyimide (PI), perfluoroalkoxy alkanes (PFA) thin films, poly(methyl methacrylate) (PMMA) sheet and ground glass were used as substrates after rinsing with ethanol and deionized water consecutively. 2.2. Photosynthesis of Metal Particles. The metal precursor was facilely prepared. NDSS, CQ and EDMAB were dissolved in ethylene glycol at a concentration of 0.066, 0.1 and 0.05 M, respectively. Then the solution was mixed with AgNO3 solution (5 M, in ethylene glycol) at a volume ratio of 1:1. A commercial ALPD (Advanced Laser Phosphor Display) laser projector emitting continuous wave laser (GaN-based, purchased from Appotronics USA, Inc.) was used after removing its color wheel. The central wavelength


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of the blue laser is 445 nm and the applied light intensity is in the range of 45-290 mW cm-2. The light intensity spectrum for ALPD system and the UV-vis absorption spectrum of CQ/EDMAB were shown in Figure S10. Ag NPs were fabricated by irradiating the precursor with projected blue laser. To learn the function of CQ/EDMAB, similar experiments were performed that the formation of Ag NPs in pure AgNO3 solution and in AgNO3 solution containing CQ/EDMAB were investigated. Pd NPs and Au NPs were synthesized in a similar way by using CQ/EDMAB as photosensitizer and CTAC as the surfactant. 2.3. Fabrication of 2D Metal Structures. Schematic illustration of the laser projection printing process was in Figure 1(Ⅰ). Photosensitive precursor, consisted of metallic salt, surfactant and photosensitizer in ethylene glycol, was pipetted into a home-made cell (the wall was silicone film and the bottom was target substrate) forming liquid layer of ~1 mm in thickness. A computer defined image was projected from the blue laser projector through the substrate onto nearby metal precursor. Upon laser exposure, the silver ions were photoreduced into Ag0 and gradually grown into Ag NPs on the substrate. After irradiation for certain time, clear silver pattern constructed by nanoparticles was formed.


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The pattern was rinsed with plenty of ethylene glycol and ethanol to remove loosely packed particles and extra reactants, and then was dried in open air. The as-prepared silver pattern was irradiated by the same laser to sinter the Ag NPs. All the processes were performed in air at room temperature. 2.4. Fabrication of 3D Metallic Objects. As shown in Figure 1(Ⅱ) and Figure S13, a desktop 3D printer equipped with the abovementioned blue laser projector was used. The precursor containing 20 phr (parts per hundred rubber by weight) AgNO3, 2 phr CQ, 2 phr EDMAB and 10 phr poly(N-vinyl pyrrolidone) (PVP) in photocurable 2-hydroxyethyl acrylate resin was put into the tank of the printer. Silver was photochemically synthesized and 2-hydroxyethyl acrylate was photopolymerized once the pre-designed laser image was projected to the precursor. The cured resin layer with the embedded Ag NPs was attached to the top translation stage. The stage was lifted up a distance of 50 μm after each irradiation so that a polymer-silver nanocomposite layer with a thickness of 50 μm was prepared. By repeating this process, 3D object was built layer by layer. To improve the electrical conductivity, the prepared 3D object was sintered at 150 °C in air for 2 hours.


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2.5. Characterization. Samples were characterized by a JSM-7500F field-emission scanning electron microscope (SEM, JEOL, Japan). Energy dispersive spectrometer (EDS) analysis was also performed at the same time. SEM images were converted to binary images and the area coverage by Ag NPs and the particle size were determined via the ImageJ software. Incident light intensity of the projector was measured by a photo power meter equipped with a 919P thermopile sensor (Newport Corporation in California). Powder X-ray diffraction (XRD) tests were performed on the Empyrean X-ray diffractometer using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific ESCALab 250Xi using 200 W monochromatic Al Kα radiation. The electron paramagnetic resonance (EPR) spectra were recorded at 100 kHz magnetic field modulation with a Bruker E500 spectrometer operating in the X-band. The microwave power and the modulate amplitude were 16 mW and 2 G, respectively. Sheet resistance was measured using a four-point probe resistivity measurement system (RTS-9, Guangzhou, China). Resistance was measured using a digital multimeter (VC890C+ VICTOR China). Transmission electron microscope (TEM) images were obtained by using JEM-2100F field emission electron microscope (JEOL,


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Japan). The UV-vis absorption spectrum was performed on a PerkinElmer Lambda 950 instrument. The light spectrum of ALPD system was measured by a spectroradiometric analysis light-measurement system (Jaz-ULM-200, Ocean Optics Inc., USA). Surface roughness was measured by atomic force microscope (AFM, MultiMode 8, Bruker, Germany).

Figure 1. Schematic illustration showing the 2D metal patterning (Ⅰ) and 3D printing ( Ⅱ ) procedures via blue laser projection processing. Ⅰ : An exemplificative “ICCAS”


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pattern is projected from a portable blue laser projector and then passes through a substrate to the photoreactive metal precursor. Upon exposure to laser, silver ions are photoreduced into Ag0 atoms that nucleate on the substrate and gradually grown into Ag NPs. After moderate laser sintering, conductive silver pattern can form, for example, on the PET thin film shown in the photo. Ⅱ: Commercial 3D printer is equipped with the blue laser projector and employed for printing 3D object. The photoreduction of silver ions and polymerization of photocurable resin happen simultaneously in the blue laser projection printing. 3D structures constructed by polymer-silver nanocomposite can be prepared and show metallic appearance after thermal sintering, such as the Great Wall model presented in the photo. The ICCAS logo is reproduced with permission from the Institute of Chemistry Chinese Academy of Sciences.

3. RESULTS AND DISCUSSION In this report, new formulations of photosensitive metal precursors are developed for laser projection printing of conductive 2D and 3D metallic structures. It had been demonstrated that CQ was an efficient dye irradiated by visible light and often used


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together with the co-initiator EDMAB.29,30 Here, CQ/EDMAB are innovatively applied to increase the efficiency of photoreduction of metal ions. At the same time, NDSS is added in the purpose of controlling silver particle size, since smaller metal nanoparticles have more moderate melting temperature31 which would benefit the laser sintering process for improving electrical conductivity. 3.1. Photosynthesis of Ag NPs Assisted by Surfactant. We studied the functions of photosensitive CQ/EDMAB and the surfactant NDSS on the synthesis of silver nanoparticles. Without CQ/EDMAB and NDSS, after irradiating AgNO3 solution in ethylene glycol for 2 hours, only a few small Ag NPs (～10 nm) were formed on the PET substrate (Figure 2A). The amount of Ag NPs was far from sufficient for metal patterning. It was reported that exposing solution of AgNO3 in ethylene glycol to high-power laser (107 mW cm-2) would lead to formation of Ag NPs, and a laser-liquid-solid interaction mechanism was proposed.32,33 In the current experiment, the laser power was much lower (102 mW cm-2) that only a small amount of Ag NPs were formed (EDS spectra shown in Figure S1-S2). However, when CQ/EDMAB were added in the precursor, Ag NP layer covering 80% of the substrate were prepared (Figure 2B). We monitored by SEM the


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fabrication of Ag NPs along with laser irradiation time increased. After 5 minutes’ exposure to laser, small particles were formed on the PET substrate (Figure 2(b.1)) similar to that shown in Figure 2(a.1). In the following, they gradually grew into particles of irregular shapes. The XRD analysis (Figure 2C) showed peaks at 38.1°, 44.3°, 64.4° and 77.4°, which were assigned to the (111), (200), (220) and (311) faces of the facecentered cubic (fcc) crystalline structure of Ag (JCPDS No. 04-0783). It is apparent that CQ/EDMAB significantly improve the efficiency of photochemical synthesis of Ag NPs under present low-power laser irradiation. Although many Ag NPs were formed, the particles were large and loosely dispersed on the substrate (Figure 2B). Conducting path could hardly be formed by sintering these dispersive and large nanoparticles.33 NDSS was then employed as surfactant, for one reason is to reduce particle size,34 which would benefit the particle sintering. For another reason, we also expected an increased particle quantity by using NDSS to improve particle nucleation. The experimental results (Figure 2D) showed that, irradiating the precursor containing both CQ/EDMAB and NDSS by laser led to nucleation and growth of Ag NPs (see also TEM images in Figure S8). Especially, small Ag NPs with diameters mostly in the range


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of 10-70 nm (size distribution shown in Figure S9) were prepared, and a higher surface coverage of about 94% was achieved. The arithmetic average roughness (Ra) of silver layer measured on a 10 μm × 10 μm area was 43.5 nm (Figure S12). In addition, XPS analysis shown in Figure S7 in SI also illustrated the NDSS component on particle surfaces. The functions of NDSS on synthesizing Ag NPs were thus proved. Besides silver, other metals can also be employed. For instance, gold (Au) and palladium (Pd) nanoparticles were photosynthesized by using K2PdCl4 and HAuCl4 as metal sources, respectively. SEM images, EDS and XRD spectra of the photosynthesized Ag, Au and Pd samples were presented in Figure S3-S5 in SI. These results well demonstrate the effectiveness of the newly developed photosensitive metal precursors for synthesizing metal nanoparticles upon exposure to laser of low power.


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Figure 2. (A) SEM images of (a.1) Ag NPs prepared on PET film from laser irradiating AgNO3 solution in ethylene glycol for 120 min and (a.2) bare PET film for comparison. EDS analyses were also performed and shown in Figure S1-S2. (B) SEM images of Ag NPs prepared from laser irradiating the precursor consisting of AgNO3 and CQ/EDMAB in ethylene glycol for, from left to right, 5min, 30 min and 120 min, respectively. (C) XRD spectrum of silver pattern on PET film. Unmarked peaks at 46.6° and 53.7° belong to PET (Figure S2 in SI). (D) SEM images showing the nucleation and growth of Ag NPs prepared


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from precursor containing both CQ/EDMAB and NDSS along with irradiation time increased (d.1-d.3: 5, 30 and 120 min, respectively). The light intensity applied was 290 mW cm-2. Scale bars are 200 nm.

3.2. Photoreduction Mechanism. As shown and discussed above that photosensitive CQ/EDMAB are quite effective in photoreduction of metal ions. However, to the best of our knowledge, photoreaction between CQ/EDMAB and metal salt has never been reported. Usually CQ/EDMAB are used to initiate polymerization of photocurable resins and CQ can absorb light under irradiation producing free radicals.29,30 Besides UV-vis absorption spectrum presented in Figure S10, we also recorded the EPR spectra of CQ/EDMAB in ethylene glycol (Figure 3A). Without light exposure, no signal was detected. Upon blue laser irradiation, EPR signal produced from camphorquinone35,36 was detected. However, the addition of AgNO3 to the solution led to disappearance of EPR signal, implying quenching of camphorquinone radical by AgNO3. Hence, based on the successful particle synthesis and the analysis, we deduced that photoreductive reaction between CQ/EDMAB and AgNO3 occurred upon exposure to blue laser.


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We propose here a possible photochemical mechanism37 as schematically illustrated in Figure 3B. Upon the blue laser exposure, CQ absorbs light to produce an excited singlet state that passes into an excited triplet state (CQ).30 CQ can abstract a hydrogen atom from EDMAB producing a hydroxycamphorquinone radical and a carbon-based amine radical. The amine radical cannot be detected by EPR since its lifetime is extremely short.38 The hydroxycamphorquinone radical is an electron-rich radical species which is also capable of undergoing oxidation by an oxidizing agent.39 Transfer of an electron from the hydroxycamphorquinone radical to the silver ion leads to formation of silver atom. The formed silver atom can react with other atoms to nucleate and the following produced atoms can grow onto the existed nuclei to form particles.


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Figure 3. (A) EPR spectra of CQ/EDMAB in ethylene glycol upon blue laser irradiation. Addition of AgNO3 in the solution resulted in disappearance of signal as indicated by the bottom line. (B) Proposed mechanism of photoreduction of silver ion by CQ/EDMAB upon blue laser irradiation and the growth of silver nanoparticles.

3.3. 2D Metal Patterning. Photosensitive metal precursors had been successfully formulated and they were printed into a diversity of metal structures on various substrates


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(see Figure 4). The metal patterning was achieved via laser projection printing process. Desired metal structures were printed precisely according to the projected laser images, as schematically illustrated in Figure 1(Ⅰ). It is noteworthy that the structured metal layer was fabricated simultaneously as a whole. This layered manufacturing process is quite different from the “point-to-point” process in DLW. Since images were digitally predesigned and then projected by a commercial projector, that almost arbitrary complex metal structure could be facilely prepared. In addition, the laser projection printing technique also exhibited good compatibility with many substrates. As shown in Figure 4AB, a complex silver circuit pattern was printed on flexible PFA thin film, as well as a selfdesigned graphic pattern was prepared on flexible and heat-sensitive PET thin film. Because the light intensity was low, damage of polymeric substrates arose from high exposure energy was avoided. Rigid substrates were also employed. For example, as shown in Figure 4C, that on PMMA sheet a complex silver pattern was printed at right and the pattern at left was simultaneously printed according to its mask image. In addition to flat substrates, the technique and material can also be applied to curved and rough substrates. Figure 4D showed wavy line pattern prepared on the curved inner surface of


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a PFA tube. Other special substrate like ground glass was used as well (Figure 4E). Besides silver that other metals, for instance, gold and palladium were also printed into tiger-shaped patterns, see Figure 4F-H. The successful printing of all the above metal patterns well proves that the laser projection printing technique and the photosensitive metal precursors are versatile and compatible with various substrates.


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Figure 4. (A) A 4.53 cm silver circuit printed on PFA film and then wrapped on a bottle. Photos of various silver patterns printed on different substrates: (B) PET thin film, (C) PMMA sheet, (D) inner surface of PFA tube (inner diameter, 3 mm), and (E) glass/ground glass slide. Photos of tiger-shaped patterns of (F) silver, (G) palladium and (H) gold, respectively. Scale bars are 0.5 cm.

We further investigated the printing resolution of this laser projection technique. Herein, the printing resolution is mainly limited by the image resolution (1280800 dpi) of the commercial projector. We printed continuous silver hexagonal mesh with line width of about 50 μm which was the finest (Figure 5A). Promisingly, finer structure can be obtained by using projector with higher resolution, and laser projectors with 4K resolution are already commercially available although the prices are high. The thickness of silver layer was estimated to be about 100 nm (Figure 5B). It was limited because the formed closepacked Ag NP layer covered the substrate that laser light could not penetrate through to continue inducing photochemical reaction. We also printed silver lines with a sequence of different resolutions (Figure 5C) on PET, PFA and PI thin film substrates. The


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resistance of silver line was measured by digital multimeter and the value decreased with the increase of line width (Figure 5D). Four-point probe resistivity measurement was also performed for large-area silver layers prepared on the different polymer substrates. The measured sheet resistance values were shown in Figure S11 in SI. The accordingly calculated electrical conductivities were all in the order of magnitude of 106 S m-1. In particular, the silver layer printed on PET film exhibited promising conductivity of at least 6.2106 S m-1, approximately one-tenth of that of bulk silver.


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Figure 5. Study of printing resolution. (A) SEM image of a fine silver structure with line width of about 50 μm. (B) SEM images showing the cross-section of silver structure printed on PET film after sintering. (C) Photo of a sequence of silver lines with length of 1.2 cm and varied width (0.2-1.2 mm) printed on PET film. (D) Resistance of the silver lines with varied widths printed on different polymer substrates.

In literatures, to increase electrical conductivity, different approaches including plasma, laser, electrical, chemical and microwave treatment have been developed to sintering metal nanoparticles.40 Among these methods, laser sintering is featured by fast local heating and is widely used. In this work, the laser sintering approach is simple and convenient that a full-screen blue laser light was projected from the projector onto the entire metal pattern. After laser exposure, Ag NPs were well melted and conducting path formed. We studied the melting behavior of Ag NPs structure as a function of laser sintering time (Figure 6A). At an exposure time of 1 min, Ag NPs already began to melt. After 5 min, most particles coalesced with each other forming interconnected necks. Until 10 min, a continuous pattern was formed. Correspondingly, sheet resistance of the


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sintered structure decreased dramatically along with exposure time increased, and reached about 3.9 Ω sq-1 (2.6106 S m-1) after 15 min (Figure 6B). The high electric conductivity and practical size of printed metal structures provide them with promising applications in electronic devices. We mounted light emitting diodes (LEDs) bulbs (blue 3.0-3.2 V and red 2.0-2.2 V) on the printed silver patterns (Figure 6C). When turned on the power, the LEDs were illuminated. Even if the flexible film was intensively bended outwards, the LEDs kept luminous (see also Video S1 in the supporting information).


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Figure 6. (A) SEM images showing the fusion of Ag NPs along with the increase of laser sintering time (a.1-a.3: 1, 5, 10 min, respectively). Scale bars are 200 nm. (B) Sheet resistance of the sintered silver structure as a function of exposure time. (C) Functioning of LEDs mounted on silver patterns. Scale bars are 0.5 cm.

3.4. 3D Printing of Conductive Metallic Objects. One of the greatest advantages of this laser projection metal printing is its use in 3D printing. By combining the in-situ photosynthesis of silver nanoparticles with the polymerization of photocurable resins,37,41 conductive 3D objects were also printed (Figure 1). In this process, CQ/EDMAB initiated the polymerization of resins.42 Without AgNO3 in the precursor, the printed object showed light yellow color (Figure 7A (i)). When AgNO3 was added, the photosynthesis of Ag NPs in polymer matrix (Figure 7C) turned the color of the object to brown (Figure 7A (ii)). After thermal sintering, the objects showed a metallic luster (Figure 7A (iii) and B). Using this method, a 52.52 cm conductive Great Wall model was built. LED operated in circuit containing the printed object was illuminated (Figure 7B). The surface sheet resistance was in the range of 1.3-50 Ω sq-1. After sintering, plenty of Ag NPs appeared on the


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surface of the conductive object while only a few were in the bulk (Figure 7D). This difference was due to the migration of near-surface silver particles to the surface and their coalescence during the sintering process.43,44 The large silver aggregates formed on the surface led to the metal silver appearance and introduced conductivity. However, the printed 3D object showed high bulk resistivity (1.15±0.62 ×105 Ωcm) due to the insulating polymeric matrix and dispersed Ag NPs in it.14 To improve the bulk conductivity, the Ag NPs amount should be increased. The preliminary studies presented here demonstrated the feasibility of using blue laser projection for 3D printing of conductive objects.

Figure 7. (A) Photos of 3D printed nuts (diagonal distance, 1.2 cm): (i) without and (ii) with AgNO3 in the precursors and (iii) after thermal sintering of sample in (ii). (B) Photo shows


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operation of LED in circuit with the printed 52.52 cm Great Wall model object. SEM images show the surface and the inside of the printed object: (C) before sintering and (D) after sintering. Scale bars are 200 nm.

4. CONCLUSIONS In conclusion, photosensitive metallic precursors have been developed for printing conductive structures via blue laser projection printing operated at low light intensity (45290 mW cm-2). Metal NPs were photosynthesized and patterned into various 2D structures on substrates including flexible polymer films. Promising electrical conductivity (6.2106 S m-1) was achieved. Combining the in-situ photosynthesis of Ag NPs with the polymerization of photocurable resins, conductive 3D objects were also prepared. The approach of using photosensitive precursors and a portable blue laser projector based laser projection printing greatly facilitates the metal patterning, which would be promisingly competitive in printed electronics industry. ASSOCIATED CONTENT


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Supporting Information Available: Summary and comparison of photoinduced metal patterning methods; EDS and XRD analysis of printed metal structures indicating the successful photosynthesis of different metals; XPS Analysis and discussions showing NDSS component on particle surface; TEM analysis of Ag NPs; histogram of Ag particle size distribution; ALPD system laser intensity spectrum and UV-vis absorption spectrum supporting the photochemical mechanism; AFM analysis giving the surface roughness of silver layer; electrical conductivity measurement; and photograph of blue laser projector equipped desktop 3D printer. A video displaying operation of LEDs mounted on printed metal structures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Authors * E-mail: [email protected] (N.Z.) * E-mail: [email protected] (J.X.) Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENTS

This work was supported by the National Key Research and Development Program of China (2016YFB1100800), the National Nature Science of Foundation of China (51733008, 51522308), the CAS Frontier Science Research Key Project (QYZDB-SSWSLH025), the Foundation of Shenzhen Scientific and Technical Innovation Free Exploration Project (No. JCYJ20170818100112531), and the Science and Technology Innovation Commission of Shenzhen (JSGG20170824112840518).

REFERENCES

(1)

Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic Transistors and Circuits with Extreme Bending Stability. Nat. Mater. 2010, 9, 1015–1022.

(2)

Peng, Z.; Ye, R.; Mann, J. A.; Zakhidov, D.; Li, Y.; Smalley, P. R.; Lin, J.; Tour, J. M. Flexible Boron-Doped Laser-Induced Graphene Microsupercapacitors. ACS

Nano 2015, 9, 5868–5875.


32

Page 33 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3)

Huang, G. W.; Feng, Q. P.; Xiao, H. M.; Li, N.; Fu, S. Y. Rapid Laser Printing of Paper-Based Multilayer Circuits. ACS Nano 2016, 10, 8895–8903.

(4)

Tybrandt, K.; Vörös, J. Fast and Efficient Fabrication of Intrinsically Stretchable Multilayer Circuit Boards by Wax Pattern Assisted Filtration. Small 2016, 12, 180– 184.

(5)

Xiong, W.; Liu, H.; Chen, Y.; Zheng, M.; Zhao, Y.; Kong, X.; Wang, Y.; Zhang, X.; Kong, X.; Wang, P.; Jiang, L. Highly Conductive, Air-Stable Silver Nanowire@Iongel Composite Films toward Flexible Transparent Electrodes. Adv.

Mater. 2016, 28, 7167–7172.

(6)

Cho, S.; Kang, S.; Pandya, A.; Shanker, R.; Khan, Z.; Lee, Y.; Park, J.; Craig, S. L.; Ko, H. Large-Area Cross-Aligned Silver Nanowire Electrodes for Flexible, Transparent, and Force-Sensitive Mechanochromic Touch Screens. ACS Nano 2017, 11, 4346–4357.


33


(7)

Page 34 of 43

Zope, K. R.; Cormier, D.; Williams, S. A. Reactive Silver Oxalate Ink Composition with Enhanced Curing Conditions for Flexible Substrates. ACS Appl. Mater.

Interfaces 2018, 10, 3830–3837.

(8)

Liu, Y. K.; Lee, M. T. Laser Direct Synthesis and Patterning of Silver Nano/microstructures on a Polymer Substrate. ACS Appl. Mater. Interfaces 2014,

6, 14576–14582.

(9)

Zhou, W.; Bai, S.; Ma, Y.; Ma, D.; Hou, T.; Shi, X.; Hu, A. Laser-Direct Writing of Silver Metal Electrodes on Transparent Flexible Substrates with High-Bonding Strength. ACS Appl. Mater. Interfaces 2016, 8, 24887–24892.

(10) Sopeña, P.; Arrese, J.; González-Torres, S.; Fernández-Pradas, J. M.; Cirera, A.; Serra, P. Low-Cost Fabrication of Printed Electronics Devices through Continuous Wave Laser-Induced Forward Transfer. ACS Appl. Mater. Interfaces 2017, 9 (35), 29412–29417.

(11) Tong, S.; Sun, J.; Yang, J. Printed Thin-Film Transistors: Research from China.

ACS Appl. Mater. Interfaces 2018, 10, 25902–25924.


34

Page 35 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Marques-Hueso, J.; Jones, T. D. A.; Watson, D. E.; Ryspayeva, A.; Esfahani, M. N.; Shuttleworth, M. P.; Harris, R. A.; Kay, R. W.; Desmulliez, M. P. Y. A Rapid Photopatterning Method for Selective Plating of 2D and 3D Microcircuitry on Polyetherimide. Adv. Funct. Mater. 2018, 1704451.

(13) Wang, B. J.; Auyeung, R. C. Y.; Kim, H.; Charipar, N. A.; Piqué, A. ThreeDimensional Printing of Interconnects by Laser Direct-Write of Silver Nanopastes.

Adv. Mater. 2010, 22, 4462–4466.

(14) Fantino, E.; Chiappone, A.; Roppolo, I.; Manfredi, D.; Bongiovanni, R.; Pirri, C. F.; Calignano, F. 3D Printing of Conductive Complex Structures with In Situ Generation of Silver Nanoparticles. Adv. Mater. 2016, 28, 3712–3717.

(15) Blasco, E.; Müller, J.; Müller, P.; Trouillet, V.; Schön, M.; Scherer, T.; BarnerKowollik, C.; Wegener, M. Fabrication of Conductive 3D Gold-Containing Microstructures via Direct Laser Writing. Adv. Mater. 2016, 28, 3592–3595.


35


Page 36 of 43

(16) Sochol, R. D.; Sweet, E.; Glick, C. C.; Wu, S. Y.; Yang, C.; Restaino, M.; Lin, L. 3D Printed Microfluidics and Microelectronics. Microelectron. Eng. 2018, 189, 52– 68.

(17) Zhang, Y.; Zhang, F.; Yan, Z.; Ma, Q.; Li, X.; Huang, Y.; Rogers, J. A. Printing, Folding and Assembly Methods for Forming 3D Mesostructures in Advanced Materials. Nat. Rev. Mater. 2017, 2 (17019).

(18) Hirt, L.; Reiser, A.; Spolenak, R.; Zambelli, T. Additive Manufacturing of Metal Structures at the Micrometer Scale. Adv. Mater. 2017, 29, 1604211.

(19) Shukla, S.; Vidal, X.; Furlani, E. P.; Swihart, M. T.; Kim, K. T.; Yoon, Y. K.; Urbas, A.; Prasad, P. N. Subwavelength Direct Laser Patterning of Conductive Gold Nanostructures by Simultaneous Photopolymerization and Photoreduction. ACS

Nano 2011, 5, 1947–1957.

(20) Xu, B. Bin; Xia, H.; Niu, L. G.; Zhang, Y. L.; Sun, K.; Chen, Q. D.; Xu, Y.; Lv, Z. Q.; Li, Z. H.; Misawa, H.; Sun, H. B. Flexible Nanowiring of Metal on Nonplanar


36

Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Substrates by Femtosecond-Laser-Induced Electroless Plating. Small 2010, 6 (16), 1762–1766.

(21) Stellacci, F.; Bauer, C. A.; Meyer-Friedrichsen, T.; Wenseleers, W.; Alain, V.; Kuebler, S. M.; Pond, S. J. K.; Zhang, Y.; Marder, S. R.; Perry, J. W. Laser and Electron-Beam Induced Growth of Nanoparticles for 2D and 3D Metal Patterning.

Adv. Mater. 2002, 14 (3), 194–198.

(22) Taormina, G.; Sciancalepore, C.; Messori, M.; Bondioli, F. 3D Printing Processes for Photocurable Polymeric Materials: Technologies, Materials, and Future Trends. J. Appl. Biomater. Funct. Mater. 2018, 16 (3), 151–160.

(23) Fantino, E.; Chiappone, A.; Calignano, F.; Fontana, M.; Pirri, F.; Roppolo, I. In Situ Thermal Generation of Silver Nanoparticles in 3D Printed Polymeric Structures. Materials. 2016, 9, 21–23.

(24) Gonzalez, G.; Chiappone, A.; Roppolo, I.; Fantino, E.; Bertana, V.; Perrucci, F.; Scaltrito, L.; Pirri, F.; Sangermano, M. Development of 3D Printable Formulations


37


Page 38 of 43

Containing CNT with Enhanced Electrical Properties. Polymer. 2017, 109, 246– 253.

(25) Cooperstein, I.; Layani, M.; Magdassi, S. 3D Printing of Porous Structures by UVCurable O/W Emulsion for Fabrication of Conductive Objects. J. Mater. Chem. C 2015, 3, 2040–2044.

(26) Foulon, F.; Green, M.; Lawes, R. A.; Baker, J.; Goodall, F. N.; Arthur, G. Laser Projection Patterned Processing of Semiconductors. Appl. Surf. Sci. 1992, 54, 291–297.

(27) An, K.; Hong, S.; Han, S.; Lee, H.; Yeo, J.; Ko, S. H. Selective Sintering of Metal Nanoparticle Ink for Maskless Fabrication of an Electrode Micropattern Using a Spatially Modulated Laser Beam by a Digital Micromirror Device. ACS Appl.

Mater. Interfaces 2014, 6 (4), 2786–2790.

(28) Yeo, J.; Hong, S.; Lee, D.; Hotz, N.; Lee, M. T.; Grigoropoulos, C. P.; Ko, S. H. Next Generation Non-Vacuum, Maskless, Low Temperature Nanoparticle Ink


38

Page 39 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Laser Digital Direct Metal Patterning for a Large Area Flexible Electronics. PLoS

One 2012, 7 (8), 1–9.

(29) Cook, W. D. Photopolymerization Kinetics of Dimethacrylates Using the Camphorquinone/amine Initiator System. Polymer. 1992, 33, 600–609.

(30) Schroeder, W. F.; Asmussen, S. V.; Sangermano, M.; Vallo, C. I. Visible Light Polymerization of Epoxy Monomers Using an Iodonium Salt with Camphorquinone/ethyl-4-Dimethyl Aminobenzoate. Polym. Int. 2013, 62, 1368– 1376.

(31) Buffat, P.; Borel, J. P. Size Effect on the Melting Temperature of Gold Particles.

Phys. Rev. A 1976, 13 (6), 2287–2298.

(32) Poondi, D.; Subramanian, R.; Otooni, M.; Singh, J. Synthesis of Silver Nanoparticles by a Laser-Liquid-Solid Interaction Technique. J. Mater. Synth.

Process. 1998, 6, 89–104.


39


Page 40 of 43

(33) Poondi, D.; Dobbins, T.; Singh, J. A Novel Laser-Liquid-Solid Interaction Technique for Synthesis of Silver, Nickel and Immiscible Silver-Nickel Alloys from Liquid Precursors. J. Mater. Sci. 2000, 35, 6237–6243.

(34) Cao, Y. Y.; Takeyasu, N.; Tanaka, T.; Duan, X. M.; Kawata, S. 3D Metallic Nanostructure Fabrication by Surfactant-Assisted Multiphoton-Induced Reduction.

Small 2009, 5 (10), 1144–1148.

(35) S. Sur, J. P. C. and J. K. S. W. Optically Active Free Radicals: Circular Dichroism, Optical Absorption and Emission and ESR Studies on Optically Active 1S(+)- and 1R(-)-Camphorquinone Radicals. Chem. Phys. 1986, 108 (3), 133–139.

(36) Monroe, B. M.; Weiner, S. A. Mechanisms of Photochemical Reactions in Solution. LVIII. Photoreduction of Camphorquinone. J. Am. Chem. Soc. 1969, 91 (2), 450–456.

(37) Dadashi-Silab, S.; Doran, S.; Yagci, Y. Photoinduced Electron Transfer Reactions for Macromolecular Syntheses. Chem. Rev. 2016, 116, 10212–10275.


40

Page 41 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Schroeder, W. F.; Cook, W. D.; Vallo, C. I. Photopolymerization of N,NDimethylaminobenzyl Alcohol as Amine Co-Initiator for Light-Cured Dental Resins. Dent. Mater. 2008, 24, 686–693.

(39) Crivello, J. V. A New Visible Light Sensitive Photoinitiator System for the Cationic Polymerization of Epoxides. J. Polym. Sci. Part A Polym. Chem. 2008, 47, 866– 875.

(40) Wünscher, S.; Abbel, R.; Perelaer, J.; Schubert, U. S. Progress of Alternative Sintering Approaches of Inkjet-Printed Metal Inks and Their Application for Manufacturing of Flexible Electronic Devices. J. Mater. Chem. C 2014, 2, 10232– 10261.

(41) Page, Z. A.; Narupai, B.; Pester, C. W.; Bou Zerdan, R.; Sokolov, A.; Laitar, D. S.; Mukhopadhyay, S.; Sprague, S.; McGrath, A. J.; Kramer, J. W.; Trefonas, P.; Hawker, C. J. Novel Strategy for Photopatterning Emissive Polymer Brushes for Organic Light Emitting Diode Applications. ACS Cent. Sci. 2017, 3, 654–661.


41


Page 42 of 43

(42) Cui, K. J.; Zhu, C. Z.; Zhang, H.; Xuan, Q.; Zou, W. Z.; Zhang, Z. Y.; Lin, X. C.; Zhao, N.; Xu, J. Blue Laser Diode-Initiated Photosensitive Resins for 3D Printing.

J. Mater. Chem. C 2017, 5, 12035–12038.

(43) Wu, Z.; Wu, D.; Zhang, T.; Jiang, L.; Wang, W.; Jin, R. Preparation of Silvered Polyimide Films with Reflective and Electrically Conductive Surfaces Using Single-Stage Self-Metallization Techniques and Factors Affecting Silver Particles Migration. J. Appl. Polym. Sci. 2006, 102, 2218–2225.

(44) Lu, G.; Yang, W.; Mei, Y.; Li, X.; Chen, G.; Chen, X. Mechanism of Migration of Sintered Nanosilver at High Temperatures in Dry Air for Electronic Packaging.

IEEE Trans. Device Mater. Reliab. 2014, 14 (1), 311–317.


42

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Table of Contents


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Blue Laser Projection Printing of Conductive Complex 2D and 3D

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